To be fair, I agree they're not bad characters, I just feel like they're among the many un-necessary characters that have been thrown into the mix over the years. It also doesn't help that, up until 2005, for whatever reason, my PBS would skip Season 6 altogether, and for the longest time, I never even saw the episode where the Molinas moved in next door, so whenever I'd Alberto or Vicita, I would wonder where they'd come from. Same with Marina for the longest time, but the episode "Tardy Tumbler" did a world of good for her.
Now, Ladonna, I'm mixed about. On the one hand, part of me wants to automatically hate her because, again, she was thrown into the mix, so late into the series' run, and as a major/main character at that (with Bud also sort of becoming D.W.'s sidekick) . . . and, I know a lot of fans actually do see her as a Scrappy. On the other hand, I can't bring myself to hate her, because . . . she's not really a bad character, albeit a little annoying at times (particularly in her debut episode) . . . and being a southerner myself, I suppose she's relatable in some cases (such as her desire to experience an actual snowy winter, her love of storytelling, and the like). And now, she and her family are gone, so . . . again, their addition and inclusion was rather pointless.